1. Field of the Invention
The present invention relates to a storage system, and more particularly, to a storage system that connects to a computer system requiring high availability.
2. Description of the Related Art
A present-day computer system, combined with electronic commerce, is required to run continuously, —24 hours a day, 365 days a year. The computer system is also required to ensure integrity as the amount of information to be maintained increases every year. Among these required functions is the data backup function that immediately recovers data that might be lost by device errors.
In general, data recorded on an external storage system (storage system) such as a hard disk is copied regularly to a tape and so on for saving (creating a backup) thereon to allow lost data to be recovered in the event of a device error, software error, or operation error. At this time, it is required to save backup data in the precise image at the time it is saved and to continue regular online processing while performing backup processing. To do so, data accessed online is duplexed to some other volume, and backup processing is performed for data on the duplexed volume while online processing is performed. Online processing during backup processing is performed with update data on the other volume.
Conventionally, a program running on a host computer has managed backup processing like this.
For example, U.S. Pat. No. 5,649,152 discloses a technology for allowing a backup program in the host computer to execute all necessary operations—management of data duplexing, data saving at a specific point in time, instruction of data backup to some other storage system, and so on.
U.S. Pat. No. 5,051,887 discloses a technology for continually creating a dual copy of data in the controller under control of a host computer. However, this method gives a heavy load to the data transfer path between the host computer and a controller.
One of data duplexing methods for use in an external storage system is to perform backup management, conventionally done by a program running in a computer, in the external storage system. An example of this technology is disclosed in U.S. Pat. 5,845,295. According to this technology, duplexing may be managed, and data may be saved, in an external storage system. When creating a backup copy of data, the host computer issues a READ (hereinafter RD) request to the external storage system, reads data from it, and then issues a WRITE (hereinafter WR) request to some other storage system to write the data. That is, the external storage system simply transfers data to a host computer in response to an instruction from the host computer.
Subsequently, U.S. Pat. No. 5,742,792 discloses a technology for copying data to some other storage system, not by an instruction from the host system, but directly by an external storage system. This method allows data to be copied in the background while performing online processing. This technology, applicable also to backup processing, may be used to execute data backup processing while executing long continuous operation.
As described in the prior art, the technology for executing data backup processing while executing long continuous operation is already established. However, external storage systems used in the disclosed technology are large external storage systems. When executing data backup processing and online processing, the data transfer portion of a large external storage system, that is, the data connection portion connected to a host computer and the connection portion (hereinafter called port) of a backup unit are implemented by separate hardware units. This is because a plurality of hardware units are installed in a large external storage system to allow redundancy. The prior art is applicable to the large external storage system described above.
However, a small external storage system connected to a workstation or a personal computer, if redundant in configuration, increases the cost and does not bring out an advantage offered by a small-sized system. Like a large external storage system, a need arises for a small external storage system to execute backup processing while executing long continuous operation. A first problem to solve is how to meet this need with less hardware.
In addition, if data is updated during duplexing in a conventional backup method in which an instruction is given by the host, the update of data during duplexing requires four times of transfer operation between the external storage system and the host computer (two WRs(Write) for duplexing, RD(Read) for backup, and WR(Write) for another storage system). This increases the bus load between the host computer and the storage system, affecting the performance of daily applications.
In particular, the backup operation of a large amount of data, which requires the processor in the external storage system to spend much time in the backup operation, degrades application performance for a long time until the backup operation ends. Recently, although more and more external storage systems employ the duplexing function, this function sometimes degrades the performance of daily online processing during backup processing. In addition, a small- to medium-size disk array controller for an open systems environment has only one data transfer path between the controller and the host computer. In a system using this path as a bus, the daily online processing must be stopped to execute backup operation.